Inhibition kinetics of nitrate uptake by ammonium in size-fractionated oceanic phytoplankton communities: Implications for new production and f-ratio estimates

Abstract

Ammonium inhibition of nitrate uptake by natural phytoplankton communities was investigated in two size fractions (<2 and >2 μm) in the northeast basin of the Atlantic Ocean. Kinetics of the inhibition process were assessed in surface waters by measuring nitrate uptake rates over a range of ammonium concentrations with 15N tracer techniques. Extremely low levels of ammonium were shown to significantly inhibit nitrate uptake in both small and large fractions at all seven sampling stations. The half-inhibition constant was significantly lower and the maximum inhibition was generally higher in the <2 μm (Ki = 60 ± 29 nmol L-1, Imax = 0.74 ± 0.13) compared to the >2 μm size fractions (Ki = 132 ± 72 nmol L-1, Imax = 0.67 ± 0.12), demonstrating that the ammonium inhibition was more pronounced in the small size class. Based on the surface ammonium concentrations of the study area, inhibition kinetics parameters predict a mean reduction of nitrate uptake of 27.8% (±12.6) for <2 μm fraction and of 16.5% (± 12.8) for >2 μm fraction which leads to a mean reduction of 20.8% (± 11.9) for the total community. The importance of the inhibition size specificity in regulating new production and the f-ratio is discussed. © The Author 2008. Published by Oxford University Press. All rights reserved.